Folic acid is a water-soluble essential B vitamin found in many foods, including leafy green vegetables, citrus fruits, and beans. Research has established that supplementation with folic acid before and during the first two months of pregnancy can prevent neural tube defects in developing fetuses in women. The U.S. Public Health Service has recommended that women of childbearing age take a supplement containing 400 mcg of folic acid every day to reduce the risk of a pregnancy affected by neural tube defects. In addition to its prevention of serious birth defects, folic acid plays a key role in the metabolism of homocysteine. Homocysteine is a sulfur containing amino acid that is created in the body from methionine, an essential amino acid derived solely from dietary intake. High plasma levels of homocysteine appear to injure the vasculature, impairing the functional abilities of endothelial and smooth muscle cells. Elevated homocysteine also appears to be thrombogenic. Suboptimal intake of several B vitamins, including folic acid, in addition to renal failure and genetic defects in homocysteine metabolism, can all contribute to abnormal homocysteine levels.